The present invention relates to an optical information recording/reproducing optical system and an optical information recording/reproducing apparatus for recording information to and/or reproducing information from an optical disc based on a predetermined standard by irradiating an information recording layer of the optical disc with a laser beam, and particularly to an optical information recording/reproducing optical system and an optical information recording/reproducing apparatus suitable for information recording/reproducing for a high-recording density optical disc, such as BD (Blu-ray Disc).
There exist various standards of optical discs, such as CD (Compact Disc) and DVD (Digital Versatile Disc), differing in recording density, protective layer thickness, etc. Meanwhile, high-recording density optical discs (e.g., BD), having still higher recording density than DVD, are being brought into practical use in recent years to realize still higher information storage capacity. Incidentally, in this specification, the “optical information recording/reproducing apparatuses” include apparatuses for both information reproducing and information recording, apparatuses exclusively for information reproducing, and apparatuses exclusively for information recording, and the same thing can be said for the “optical information recording/reproducing optical systems”. In the following, the “optical information recording/reproducing optical system” is frequently referred to as an optical system of an optical pick-up.
When information recording or information reproducing for an optical disc is executed in the optical information recording/reproducing apparatus, undesired reflected light or scattered light, which are generated on optical surfaces of optical elements (e.g., an objective lens and a collimator lens configuring an optical system of an optical pick-up) may interfere with signal light, and thereby deteriorate the quality of the signal light which is used for information recording or information reproducing. For this reason, in many cases, a functional film, such as an antireflection film or an antistatic film for preventing adhesion of dust to an optical surface, is provided on an optical surface of an optical element (e.g., an objective lens) configuring an optical system of an optical pick-up.
Incidentally, in order to execute information recording or information reproducing for a high-recording density optical disc, such as BD, it is required to converge a laser beam to have a small beam spot diameter on the recording layer of the optical disc. For this reason, a laser beam having a short wavelength, for example, around 400 nm (hereafter, referred to as “blue laser light”) is used, and an objective lens having a larger numerical aperture (e.g., 0.85 for BD) and having a short focal length is used. Since a lens surface of an objective lens of this type has a small curvature radius, an incident angle of the laser beam on the lens surface of the objective lens becomes larger at a point closer to the periphery of the objective lens, and therefore the reflectivity of the lens surface becomes larger at a point closer to the periphery of the objective lens. As a result, a problem arises that the rim intensity (a ratio of the intensity of light passing through a peripheral part of an objective lens to the intensity of light passing through a central part close to an optical axis of the objective lens) of the laser beam which has passed through the objective lens decreases, and thereby the periphery of the beam spot converged on the recording layer is blurred and the entire power of the beam spot also decreases.
Japanese Patent Provisional Publication No. 2008-217886A (hereafter, referred to as JP2008-217886A) discloses a technique for decreasing the beam spot diameter on the recording layer of the optical disc by providing an antireflection film on an entrance surface of an objective lens having a small curvature radius. Specifically, the antireflection film has a low transmissivity in the central portion of the objective lens and has a high transmissivity in the peripheral part of the objective lens.
Since the blue laser beam used for information recording or the information reproducing for the high-recording density optical disc has high energy, the blue laser beam tends to chemically affect the base material of an optical element and thereby alter the quality of the base material of the optical element. Therefore, if a resin lens with a coating is irradiated with the blue laser beam having a power density larger than or equal to a certain level for a long time, minute change in shape occurs on the optical surface of the resin lens, and thereby the optical performance (e.g., aberrations or the optical efficiency) of the resin lens deteriorates. Japanese Patent Provisional Publication No. 2005-251354A (hereafter, referred to as JP2005-251354A) suggests changing the resin base material of an optical element, as a measure against the above described problem.
However, it is difficult to control the reflectivity distribution of an antireflection film with a high degree of positional accuracy, particularly on an optical surface having a complicated three-dimensional shape, such as an optical surface of an objective lens. Therefore, realizing the antireflection film disclosed in JP2008-217886A on an optical surface having a complicated three-dimensional shape with currently available processing technology is difficult. Furthermore, it is difficult to achieve the transmissivity distribution suggested in JP2008-217886A without using coating material which tends to cause the change in shape on the base material of an optical element when the optical element is irradiated with the blue laser beam.
More specifically, in JP2005-251354A, a technique in which resin having an alicyclic structure is used as the resin base material to prevent decreasing of the degree of adhesiveness with respect to a functional film is suggested. However, it should be noted that the resin material is one of most important optical design parameters defining the optical performance of the optical element (i.e., the optical performance of an optical system of an optical pick-up). Therefore, it is not desirable to limit a selection range of resin materials. That is because the degree of design freedom reduces. In actuality, an optical configuration suggested in JP2005-251354A is not able to realize the optical performance required for an optical system of an optical pick-up for BD. In addition, the optical configuration suggested in JP2005-251354A is not able to achieve a sufficient light-resisting property although the configuration is able to extend, to a certain extent, the time that elapses before the change in shape occurs on the coated surface of the optical element.